Selecting the right head for what you are doing is very important. The GT40X heads are OK but found them to be not up to what I like out of my engines. The work very well on a mild 302. for a mid-range 302 from 350hp to 400hp the Edelbrock Performer RPM works very well. If you want over 400hp and will spin the engine pretty hard or are using a stroker the Victor Jr's and Brodix track 1 heads I found are the best for the money. Those heads really need 11-1 compression and a very aggressive cam to work to there full potential.
bolting a set of big heads to a mild engine will do what Ron said. Bolting small heads on a big engine will give you lots of torque in the low engine and mid-range but it will fall on it's face at 4000 revs.
I think what it all boils down to is flow rate and velocity. If you put heads on an engine that cannot pull enough air through their big open ports to keep the velocity of the flow up, it will spin well but won't have any torque. An engine with enough displacement and cam lift and duration to move lots of air will provide a lot of velocity through small ported heads (torque)but not enough flow for higher RPMs. It is a bit of an understatement, but torque does relate to the velocity of the intake charge and RPM to the flow rate. There are other factors like peak flow vs flow at mid lift; like Ron said once "its the total area under the curve not the curve's peak value." So an advertisement for heads that only talks about peak flow isn't telling the full story.
What is the flow rate when the valve is half open? (The valve spends a lot more time around the half open position than at the full open position, so flow at this point has a more of a direct relationship to how well the heads breathe.) If you can balance the two - flow and velocity (read lots of both), LOOK OUT! And then there is intake/exhaust overlap and there are the sinotic pressure pulses in the intake that are the subject of "tuning" the precise length of runners (and headers). And short runners for RPM and long runners for torque to trap the bounce back of the air's velocity when the valve slams shut... and so on and so on...... and eventually it becomes an art where guys like Levy and others excell because, knowing all of this, they have tried most of the combinations that make sense and can feel when it's right.
Since I talked my self in a circle, I'll shut up now!
I have not had experience with CNC ported heads but do know that in Perth the "Kostecki Engine Centre" are supposed to be THE people with THE equipment. They are apparently doing a number of heads for leading V8 supercar teams but not only that, they are lightening the blocks by CNC as well to shave off vital kg's
Can't help with design or purpose on all 3, but I have used all 3 in Mustangs and can tell you a little bit about emperical findings.
The alloy GT40 head, the first one, with the Y in the serial number was not that great. I think some well ported E7 stockers would flow as good as these. Make sure you do not use the 65cc chamber vesion and suffer from lower compression. Or mill it.
The X version is pretty good, it flows better than the first version. Good mid-range and top end. I liked these on a 3500 lb car and a blower.
The Victor Jr. out flows either of these heads but needs a proper cam and induction to take advantage of it. Otherwise, bolting them on a mild engine will lead to losses in the low and mid range (been there, done that).
I'd select the Victor Jr. with a strong small block and good induction. The short gearing and a good cam will make the work well.
Coast has a good crate motor (302,331,342) with CNC ported Victor Jrs that flow a lot. get a good cam, good valve springs, all forged internals with H beams, girdle, and get to it.